1. Technical Field
The present invention generally relates to gears for use in power transmission assemblies such as motor vehicle transmissions and transfer cases. More particularly, the present invention relates to a multi-piece gear assembly having reduced weight and rotational inertia which can be economically manufactured for replacement of conventional monoblock gears.
2. Discussion
Gear manufacture is a time-consuming and expensive process. Much of the cost associated with gear manufacturing arises from the cost of the gear material. Traditionally, gears are constructed from a homogeneous gear blank obtained from a billet of high alloy gear steel such as, for example, SAE 8620. While superior mechanical properties such as hardenability and fatigue strength are desirable in the gear tooth portion of such monoblock gear, these particular mechanical properties are not required throughout the remainder of the gear. In addition, when a gear is fabricated from a single material an undesirably large rotational moment of inertia results. As the rotational moment of inertia increases, so does the amount of energy required to start and stop rotation of the gear. Accordingly, a gear having a reduced rotational moment of inertia is desirable for production of an energy efficient power transmission device.
While gears having a reduced rotational moment of inertia are known in the art, such designs have certain disadvantages. For example, it is known to machine a relief into the central web portion of the gear in order to reduce its weight and rotational moment of inertia. While this method accomplishes the task of reducing the rotational moment of inertia, the overall gear design may be compromised. Specifically, the remaining web section may have been reduced to the point that subsequent heat treatment operations will cause deformation of the gear body and tooth form. This may result in a need for hard finishing of the gear form thus adding significant costs.
Other methods employed to reduce the weight and rotational moment of inertia of gears include utilization of different materials. In this regard, some composite materials such as fiber reinforced plastic, have been successful in some light duty applications. However, the heat and stress induced on the teeth of gears used in power transmission assemblies, such as motor vehicle transmissions and transfer cases, exceeds the allowable limits for most known composite materials. As such, composite gears have had only limited application in motor vehicles.